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.Dd October 6, 2025
.Dt FTS 3
.Os
.Sh NAME
.Nm fts
.Nd traverse a file hierarchy
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In fts.h
.Ft FTS *
.Fn fts_open "char * const *path_argv" "int options" "int (*compar)(const FTSENT * const *, const FTSENT * const *)"
.Ft FTS *
.Fn fts_open_b "char * const *path_argv" "int options" "int (^compar)(const FTSENT * const *, const FTSENT * const *)"
.Ft FTSENT *
.Fn fts_read "FTS *ftsp"
.Ft FTSENT *
.Fn fts_children "FTS *ftsp" "int options"
.Ft int
.Fn fts_set "FTS *ftsp" "FTSENT *f" "int options"
.Ft void
.Fn fts_set_clientptr "FTS *ftsp" "void *clientdata"
.Ft void *
.Fn fts_get_clientptr "FTS *ftsp"
.Ft FTS *
.Fn fts_get_stream "FTSENT *f"
.Ft int
.Fn fts_close "FTS *ftsp"
.Sh DESCRIPTION
The
.Nm
functions are provided for traversing
.Ux
file hierarchies.
A simple overview is that the
.Fn fts_open
and
.Fn fts_open_b
functions return a
.Dq handle
on a file hierarchy, which is then supplied to
the other
.Nm
functions.
The
.Fn fts_read
function returns a pointer to a structure describing one of the files
in the file hierarchy.
The
.Fn fts_children
function returns a pointer to a linked list of structures, each of
which describes one of the files contained in a directory in the
hierarchy.
In general, directories are visited two distinguishable times; in pre-order
(before any of their descendants are visited) and in post-order (after all
of their descendants have been visited).
Files are visited once.
It is possible to walk the hierarchy
.Dq logically
(ignoring symbolic links)
or physically (visiting symbolic links), order the walk of the hierarchy or
prune and/or re-visit portions of the hierarchy.
.Pp
Two structures are defined (and typedef'd) in the include file
.In fts.h .
The first is
.Vt FTS ,
the structure that represents the file hierarchy itself.
The second is
.Vt FTSENT ,
the structure that represents a file in the file
hierarchy.
Normally, an
.Vt FTSENT
structure is returned for every file in the file
hierarchy.
In this manual page,
.Dq file
and
.Dq Vt FTSENT No structure
are generally
interchangeable.
.Pp
The
.Vt FTS
structure contains space for a single pointer, which may be used to
store application data or per-hierarchy state.
The
.Fn fts_set_clientptr
and
.Fn fts_get_clientptr
functions may be used to set and retrieve this pointer.
This is likely to be useful only when accessed from the sort
comparison function, which can determine the original
.Vt FTS
stream of its arguments using the
.Fn fts_get_stream
function.
The two
.Li get
functions are also available as macros of the same name.
.Pp
The
.Vt FTSENT
structure contains at least the following fields, which are
described in greater detail below:
.Bd -literal
typedef struct _ftsent {
	int fts_info;			/* status for FTSENT structure */
	char *fts_accpath;		/* access path */
	char *fts_path;			/* root path */
	size_t fts_pathlen;		/* strlen(fts_path) */
	char *fts_name;			/* file name */
	size_t fts_namelen;		/* strlen(fts_name) */
	long fts_level;			/* depth (\-1 to N) */
	int fts_errno;			/* file errno */
	long long fts_number;		/* local numeric value */
	void *fts_pointer;		/* local address value */
	struct ftsent *fts_parent;	/* parent directory */
	struct ftsent *fts_link;	/* next file structure */
	struct ftsent *fts_cycle;	/* cycle structure */
	struct stat *fts_statp;		/* stat(2) information */
} FTSENT;
.Ed
.Pp
These fields are defined as follows:
.Bl -tag -width "fts_namelen"
.It Fa fts_info
One of the following values describing the returned
.Vt FTSENT
structure and
the file it represents.
With the exception of directories without errors
.Pq Dv FTS_D ,
all of these
entries are terminal, that is, they will not be revisited, nor will any
of their descendants be visited.
.Bl  -tag -width FTS_DEFAULT
.It Dv FTS_D
A directory being visited in pre-order.
.It Dv FTS_DC
A directory that causes a cycle in the tree.
(The
.Fa fts_cycle
field of the
.Vt FTSENT
structure will be filled in as well.)
.It Dv FTS_DEFAULT
Any
.Vt FTSENT
structure that represents a file type not explicitly described
by one of the other
.Fa fts_info
values.
.It Dv FTS_DNR
A directory which cannot be read.
This immediately follows
.Dv FTS_D ,
in place of
.Dv FTS_DP ,
when the directory could not be entered, or could be entered but not
read.
This is an error return, and the
.Fa fts_errno
field will be set to indicate what caused the error.
.It Dv FTS_DOT
A file named
.Ql .\&
or
.Ql ..\&
which was not specified as a file name to
.Fn fts_open
or
.Fn fts_open_b
(see
.Dv FTS_SEEDOT ) .
.It Dv FTS_DP
A directory being visited in post-order.
The contents of the
.Vt FTSENT
structure will be unchanged from when
the directory was visited in pre-order, except for the
.Fa fts_info
field.
.It Dv FTS_ERR
This is an error return, and the
.Fa fts_errno
field will be set to indicate what caused the error.
.It Dv FTS_F
A regular file.
.It Dv FTS_NS
A file for which no
.Xr stat 2
information was available.
The contents of the
.Fa fts_statp
field are undefined.
This is an error return, and the
.Fa fts_errno
field will be set to indicate what caused the error.
.It Dv FTS_NSOK
A file for which no
.Xr stat 2
information was requested.
The contents of the
.Fa fts_statp
field are undefined.
.It Dv FTS_SL
A symbolic link.
.It Dv FTS_SLNONE
A symbolic link with a non-existent target.
The contents of the
.Fa fts_statp
field reference the file characteristic information for the symbolic link
itself.
.El
.It Fa fts_accpath
A path for accessing the file from the current directory.
.It Fa fts_path
The path for the file relative to the root of the traversal.
This path contains the path specified to
.Fn fts_open
or
.Fn fts_open_b
as a prefix.
.It Fa fts_pathlen
The length of the string referenced by
.Fa fts_path .
.It Fa fts_name
The name of the file.
.It Fa fts_namelen
The length of the string referenced by
.Fa fts_name .
.It Fa fts_level
The depth of the traversal, numbered from \-1 to N, where this file
was found.
The
.Vt FTSENT
structure representing the parent of the starting point (or root)
of the traversal is numbered
.Dv FTS_ROOTPARENTLEVEL
(\-1), and the
.Vt FTSENT
structure for the root
itself is numbered
.Dv FTS_ROOTLEVEL
(0).
.It Fa fts_errno
Upon return of a
.Vt FTSENT
structure from the
.Fn fts_children
or
.Fn fts_read
functions, with its
.Fa fts_info
field set to
.Dv FTS_DNR ,
.Dv FTS_ERR
or
.Dv FTS_NS ,
the
.Fa fts_errno
field contains the value of the external variable
.Va errno
specifying the cause of the error.
Otherwise, the contents of the
.Fa fts_errno
field are undefined.
.It Fa fts_number
This field is provided for the use of the application program and is
not modified by the
.Nm
functions.
It is initialized to 0.
.It Fa fts_pointer
This field is provided for the use of the application program and is
not modified by the
.Nm
functions.
It is initialized to
.Dv NULL .
.It Fa fts_parent
A pointer to the
.Vt FTSENT
structure referencing the file in the hierarchy
immediately above the current file, i.e., the directory of which this
file is a member.
A parent structure for the initial entry point is provided as well,
however, only the
.Fa fts_level ,
.Fa fts_number
and
.Fa fts_pointer
fields are guaranteed to be initialized.
.It Fa fts_link
Upon return from the
.Fn fts_children
function, the
.Fa fts_link
field points to the next structure in the NULL-terminated linked list of
directory members.
Otherwise, the contents of the
.Fa fts_link
field are undefined.
.It Fa fts_cycle
If a directory causes a cycle in the hierarchy (see
.Dv FTS_DC ) ,
either because
of a hard link between two directories, or a symbolic link pointing to a
directory, the
.Fa fts_cycle
field of the structure will point to the
.Vt FTSENT
structure in the hierarchy that references the same file as the current
.Vt FTSENT
structure.
Otherwise, the contents of the
.Fa fts_cycle
field are undefined.
.It Fa fts_statp
A pointer to
.Xr stat 2
information for the file.
.El
.Pp
A single buffer is used for all of the paths of all of the files in the
file hierarchy.
Therefore, the
.Fa fts_path
and
.Fa fts_accpath
fields are guaranteed to be
.Dv NUL Ns -terminated
.Em only
for the file most recently returned by
.Fn fts_read .
To use these fields to reference any files represented by other
.Vt FTSENT
structures will require that the path buffer be modified using the
information contained in that
.Vt FTSENT
structure's
.Fa fts_pathlen
field.
Any such modifications should be undone before further calls to
.Fn fts_read
are attempted.
The
.Fa fts_name
field is always
.Dv NUL Ns -terminated .
.Ss Thread Safety
The
.Nm
functions can safely be used in multi-threaded programs provided no
two threads access the same
.Vt FTS
or
.Vt FTSENT
structure simultaneously.
However, unless the
.Dv FTS_NOCHDIR
flag was passed to
.Fn fts_open
or
.Fn fts_open_b ,
calls to
.Fn fts_read
and
.Fn fts_children
may change the current working directory, which will affect all
threads.
Conversely, changing the current working directory either during or
between calls to
.Fn fts_read
or
.Fn fts_children
(even in a single-thread program) may cause
.Nm
to malfunction unless the
.Dv FTS_NOCHDIR
flag was passed to
.Fn fts_open
or
.Fn fts_open_b
and all paths in
.Va path_argv
were absolute.
.Ss Fn fts_open
The
.Fn fts_open
function takes a pointer to an array of character pointers naming one
or more paths which make up a logical file hierarchy to be traversed.
The array must be terminated by a
.Dv NULL
pointer.
.Pp
There are
a number of options, at least one of which (either
.Dv FTS_LOGICAL
or
.Dv FTS_PHYSICAL )
must be specified.
The options are selected by
.Em or Ns 'ing
the following values:
.Bl -tag -width "FTS_COMFOLLOWDIR"
.It Dv FTS_COMFOLLOW
This option causes any symbolic link specified as a root path to be
followed immediately whether or not
.Dv FTS_LOGICAL
is also specified.
.It Dv FTS_COMFOLLOWDIR
This option is similar to
.Dv FTS_COMFOLLOW ,
but only follows symbolic links to directories.
.It Dv FTS_LOGICAL
This option causes the
.Nm
routines to return
.Vt FTSENT
structures for the targets of symbolic links
instead of the symbolic links themselves.
If this option is set, the only symbolic links for which
.Vt FTSENT
structures
are returned to the application are those referencing non-existent files.
Either
.Dv FTS_LOGICAL
or
.Dv FTS_PHYSICAL
.Em must
be provided to the
.Fn fts_open
function.
.It Dv FTS_NOCHDIR
To allow descending to arbitrary depths
(independent of
.Brq Dv PATH_MAX )
and improve performance, the
.Nm
functions change directories as they walk the file hierarchy.
This has the side-effect that an application cannot rely on being
in any particular directory during the traversal.
The
.Dv FTS_NOCHDIR
option turns off this feature, and the
.Nm
functions will not change the current directory.
Note that applications should not themselves change their current directory
and try to access files unless
.Dv FTS_NOCHDIR
is specified and absolute
pathnames were provided as arguments to
.Fn fts_open .
.It Dv FTS_NOSTAT
By default, returned
.Vt FTSENT
structures reference file characteristic information (the
.Fa statp
field) for each file visited.
This option relaxes that requirement as a performance optimization,
allowing the
.Nm
functions to set the
.Fa fts_info
field to
.Dv FTS_NSOK
and leave the contents of the
.Fa statp
field undefined.
.It Dv FTS_NOSTAT_TYPE
This option is similar to
.Dv FTS_NOSTAT ,
but attempts to populate
.Fa fts_info
based on information from the
.Fa d_type
field of
.Vt struct dirent .
.It Dv FTS_PHYSICAL
This option causes the
.Nm
routines to return
.Vt FTSENT
structures for symbolic links themselves instead
of the target files they point to.
If this option is set,
.Vt FTSENT
structures for all symbolic links in the
hierarchy are returned to the application.
Either
.Dv FTS_LOGICAL
or
.Dv FTS_PHYSICAL
.Em must
be provided to the
.Fn fts_open
function.
.It Dv FTS_SEEDOT
By default, unless they are specified as path arguments to
.Fn fts_open ,
any files named
.Ql .\&
or
.Ql ..\&
encountered in the file hierarchy are ignored.
This option causes the
.Nm
routines to return
.Vt FTSENT
structures for them.
.It Dv FTS_XDEV
This option prevents
.Nm
from descending into directories that have a different device number
than the file from which the descent began.
.El
.Pp
The
.Fa compar
argument points to a user-defined function which may be used to order
the traversal of the hierarchy.
It
takes two pointers to pointers to
.Vt FTSENT
structures as arguments and
should return a negative value, zero, or a positive value to indicate
if the file referenced by its first argument comes before, in any order
with respect to, or after, the file referenced by its second argument.
The
.Fa fts_accpath ,
.Fa fts_path
and
.Fa fts_pathlen
fields of the
.Vt FTSENT
structures may
.Em never
be used in this comparison.
If the
.Fa fts_info
field is set to
.Dv FTS_NS
or
.Dv FTS_NSOK ,
the
.Fa fts_statp
field may not either.
If the
.Fn compar
argument is
.Dv NULL ,
the directory traversal order is in the order listed in
.Fa path_argv
for the root paths, and in the order listed in the directory for
everything else.
.Ss Fn fts_open_b
The
.Fn fts_open_b
function is identical to
.Fn fts_open
except that it takes a block pointer instead of a function pointer.
The block is copied before
.Fn fts_open_b
returns, so the original can safely go out of scope or be released.
.Ss Fn fts_read
The
.Fn fts_read
function returns a pointer to an
.Vt FTSENT
structure describing a file in
the hierarchy.
Directories (that are readable and do not cause cycles) are visited at
least twice, once in pre-order and once in post-order.
All other files are visited at least once.
(Hard links between directories that do not cause cycles or symbolic
links to symbolic links may cause files to be visited more than once,
or directories more than twice.)
.Pp
If all the members of the hierarchy have been returned,
.Fn fts_read
returns
.Dv NULL
and sets the external variable
.Va errno
to 0.
If an error unrelated to a file in the hierarchy occurs,
.Fn fts_read
returns
.Dv NULL
and sets
.Va errno
appropriately.
If an error related to a returned file occurs, a pointer to an
.Vt FTSENT
structure is returned, and
.Va errno
may or may not have been set (see
.Fa fts_info ) .
Note that
.Fn fts_read
will not set
.Va errno
to 0 if called again with the same
.Fa ftsp
argument after the
.Dv FTS_STOP
flag has been set or the end of the stream has been reached.
.Pp
The
.Vt FTSENT
structures returned by
.Fn fts_read
may be overwritten after a call to
.Fn fts_close
on the same file hierarchy stream, or, after a call to
.Fn fts_read
on the same file hierarchy stream unless they represent a file of type
directory, in which case they will not be overwritten until after a call to
.Fn fts_read
after the
.Vt FTSENT
structure has been returned by the
.Fn fts_read
function in post-order.
.Ss Fn fts_children
The
.Fn fts_children
function returns a pointer to an
.Vt FTSENT
structure describing the first entry in a NULL-terminated linked list of
the files in the directory represented by the
.Vt FTSENT
structure most recently returned by
.Fn fts_read .
The list is linked through the
.Fa fts_link
field of the
.Vt FTSENT
structure, and is ordered by the user-specified comparison function, if any.
Repeated calls to
.Fn fts_children
will recreate this linked list.
.Pp
As a special case, if
.Fn fts_read
has not yet been called for a hierarchy,
.Fn fts_children
will return a pointer to the files in the logical directory specified to
.Fn fts_open
or
.Fn fts_open_b ,
i.e., the arguments specified to
.Fn fts_open
or
.Fn fts_open_b .
Otherwise, if the
.Vt FTSENT
structure most recently returned by
.Fn fts_read
is not a directory being visited in pre-order,
or the directory does not contain any files,
.Fn fts_children
returns
.Dv NULL
and sets
.Va errno
to zero.
If an error occurs,
.Fn fts_children
returns
.Dv NULL
and sets
.Va errno
appropriately.
.Pp
The
.Vt FTSENT
structures returned by
.Fn fts_children
may be overwritten after a call to
.Fn fts_children ,
.Fn fts_close
or
.Fn fts_read
on the same file hierarchy stream.
.Pp
.Em Option
may be set to the following value:
.Bl -tag -width FTS_NAMEONLY
.It Dv FTS_NAMEONLY
Only the names of the files are needed.
The contents of all the fields in the returned linked list of structures
are undefined with the exception of the
.Fa fts_name
and
.Fa fts_namelen
fields.
.El
.Ss Fn fts_set
The
.Fn fts_set
function allows the user application to determine further processing
for the file
.Fa f
of the stream
.Fa ftsp .
The
.Fn fts_set
function
returns 0 on success, and \-1 if an error occurs.
.Em Option
must be set to one of the following values:
.Bl -tag -width FTS_PHYSICAL
.It Dv FTS_AGAIN
Re-visit the file; any file type may be re-visited.
The next call to
.Fn fts_read
will return the referenced file.
The
.Fa fts_stat
and
.Fa fts_info
fields of the structure will be reinitialized at that time,
but no other fields will have been changed.
This option is meaningful only for the most recently returned
file from
.Fn fts_read .
Normal use is for post-order directory visits, where it causes the
directory to be re-visited (in both pre and post-order) as well as all
of its descendants.
.It Dv FTS_FOLLOW
The referenced file must be a symbolic link.
If the referenced file is the one most recently returned by
.Fn fts_read ,
the next call to
.Fn fts_read
returns the file with the
.Fa fts_info
and
.Fa fts_statp
fields reinitialized to reflect the target of the symbolic link instead
of the symbolic link itself.
If the file is one of those most recently returned by
.Fn fts_children ,
the
.Fa fts_info
and
.Fa fts_statp
fields of the structure, when returned by
.Fn fts_read ,
will reflect the target of the symbolic link instead of the symbolic link
itself.
In either case, if the target of the symbolic link does not exist the
fields of the returned structure will be unchanged and the
.Fa fts_info
field will be set to
.Dv FTS_SLNONE .
.Pp
If the target of the link is a directory, the pre-order return, followed
by the return of all of its descendants, followed by a post-order return,
is done.
.It Dv FTS_SKIP
No descendants of this file are visited.
The file may be one of those most recently returned by either
.Fn fts_children
or
.Fn fts_read .
.El
.Ss Fn fts_set_clientptr , Fn fts_get_clientptr
The
.Fn fts_set_clientptr
function sets the client data pointer for the stream
.Fa ftsp
to
.Fa clientdata .
The
.Fn fts_get_clientptr
function returns the client data pointer associated with
.Fa ftsp .
This can be used to pass per-stream data to the comparison function.
.Pp
For performance reasons,
.Fn fts_get_clientptr
may be shadowed by a preprocessor macro.
.Ss Fn fts_get_stream
The
.Fn fts_get_stream
function returns the
.Nm
stream associated with the file entry
.Fa f .
A typical use for this would be for a comparison function to first call
.Fn fts_get_stream
on one of its arguments, then call
.Fn fts_get_clientptr
to obtain the client data pointer, which in turn points to information
necessary to correctly order the two entries.
.Pp
For performance reasons,
.Fn fts_get_stream
may be shadowed by a preprocessor macro.
.Ss Fn fts_close
The
.Fn fts_close
function closes a file hierarchy stream
.Fa ftsp
and restores the current directory to the directory from which
.Fn fts_open
or
.Fn fts_open_b
was called to open
.Fa ftsp .
.Sh RETURN VALUES
The
.Fn fts_open
and
.Fn fts_open_b
functions return a pointer to the new
.Nm
stream on success and
.Dv NULL
on failure.
.Pp
The
.Fn fts_read
function returns a pointer to the next file entry on success, or if an
error occurs that relates specifically to that file entry.
On reaching the end of the file hierarchy, it returns
.Dv NULL
and sets the external variable
.Va errno
to 0.
On failure, it returns
.Dv NULL
and sets
.Va errno
to an appropriate non-zero value.
If called again after the
.Dv FTS_STOP
flag has been set or the end of the stream has been reached,
.Fn fts_read
returns
.Dv NULL
and leaves
.Va errno
untouched.
.Pp
The
.Fn fts_children
function returns a pointer to a linked list of file entries on
success.
On reaching the end of the file hierarchy, it returns
.Dv NULL
and sets the external variable
.Va errno
to 0.
On failure, it returns
.Dv NULL
and sets
.Va errno
to an appropriate non-zero value.
.Pp
The
.Fn fts_set
function returns 0 on success and \-1 if its
.Fa instr
argument is invalid.
.Pp
The
.Fn fts_get_clientptr
function returns the client data pointer associated with its argument,
or
.Dv NULL
if none has been set.
.Pp
The
.Fn fts_get_stream
function returns a pointer to the
.Nm
stream associated with its argument.
.Pp
The
.Fn fts_close
function
returns 0 on success, and \-1 if an error occurs.
.Sh ERRORS
The
.Fn fts_open
and
.Fn fts_open_b
functions may fail and set
.Va errno
for any of the errors specified for the library functions
.Xr open 2
and
.Xr malloc 3 .
The
.Fn fts_open_b
function may also fail and set
.Va errno
to
.Dv ENOSYS
if the blocks runtime is missing.
.Pp
The
.Fn fts_close
function may fail and set
.Va errno
for any of the errors specified for the library functions
.Xr chdir 2
and
.Xr close 2 .
.Pp
The
.Fn fts_read
and
.Fn fts_children
functions may fail and set
.Va errno
for any of the errors specified for the library functions
.Xr chdir 2 ,
.Xr malloc 3 ,
.Xr opendir 3 ,
.Xr readdir 3
and
.Xr stat 2 .
.Pp
In addition, the
.Fn fts_children ,
.Fn fts_open ,
and
.Fn fts_set
functions may fail and set
.Va errno
as follows:
.Bl -tag -width Er
.It Bq Er EINVAL
The options were invalid, or the list was empty.
.El
.Sh SEE ALSO
.Xr find 1 ,
.Xr chdir 2 ,
.Xr stat 2 ,
.Xr ftw 3 ,
.Xr qsort 3
.Sh HISTORY
The
.Nm
interface was first introduced in
.Bx 4.4 .
The
.Fn fts_get_clientptr ,
.Fn fts_get_stream ,
and
.Fn fts_set_clientptr
functions were introduced in
.Fx 5.0 ,
principally to provide for alternative interfaces to the
.Nm
functionality using different data structures.
Blocks support and the
.Dv FTS_COMFOLLOWDIR
and
.Dv FTS_NOSTAT
options were added in
.Fx 15.0
based on similar functionality in macOS.
.Sh BUGS
The
.Fn fts_open
function will automatically set the
.Dv FTS_NOCHDIR
option if the
.Dv FTS_LOGICAL
option is provided, or if it cannot
.Xr open 2
the current directory.
